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Pembrolizumab-induced autoimmune side effects of colon and pancreas in a patient with lung cancer. Immune checkpoint inhibitors have been shown to be effective for treating many carcinomas. However, the activated immune response may lead to the development of multiple immune-related adverse events, including rare immune-mediated inflammation due to autoimmune mechanisms. An 82-year-old man was diagnosed with large cell lung cancer (T1aN3M1 stage IVB) and was treated with inhibitors of the programmed cell death receptor-1, pembrolizumab. Diarrhea and melena occurred after six doses of pembrolizumab, and colonoscopy revealed mucosal inflammation of the rectum and sigmoid colon in a continuous manner, resembling the typical endoscopic findings of ulcerative colitis. Subsequently, fever and hyperamylasemia appeared, and the patient was diagnosed with pancreatitis resembling type 2 autoimmune pancreatitis on imaging tests and cytological examination, which showed infiltration of inflammatory cells, mainly neutrophils. Steroid therapy was administered and both, colitis and pancreatitis markedly improved. Here, we present a patient who developed colitis and pancreatitis after ICI treatment for advanced lung cancer. Both are thought to be due to autoimmune side effects of pembrolizumab. Although pancreatitis is a rare irAE, clinicians should be aware of the development of pancreatitis, especially in the case of irAE colitis resembling ulcerative colitis.